Read threshold voltage selection

ABSTRACT

Apparatuses and methods for read threshold voltage selection are provided. One example method can include setting a first soft read threshold voltage and a second soft read threshold voltage based on a difference between a first number of memory cells that are read as being programmed to a first state when read using a first threshold voltage and a second number of memory that are read as being programmed to the first state when read using another threshold voltage.

TECHNICAL FIELD

The present disclosure relates generally to error correction, and moreparticularly, to apparatuses and methods for read threshold voltageselection.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including volatile and non-volatilememory. Volatile memory can require power to maintain its data andincludes random-access memory (RAM), dynamic random access memory(DRAM), and synchronous dynamic random access memory (SDRAM), amongothers. Non-volatile memory can provide persistent data by retainingstored data when not powered and can include NAND flash memory, NORflash memory, read only memory (ROM), Electrically Erasable ProgrammableROM (EEPROM), Erasable Programmable ROM (EPROM), and resistance variablememory such as phase change random access memory (PCRAM), resistiverandom access memory (RRAM), and magnetoresistive random access memory(MRAM), among others.

Memory devices can be combined together to form a storage volume of amemory system such as a solid state drive (SSD). A solid state drive caninclude non-volatile memory (e.g., NAND flash memory and NOR flashmemory), and/or can include volatile memory (e.g., DRAM and SRAM), amongvarious other types of non-volatile and volatile memory.

An SSD can be used to replace hard disk drives as the main storagevolume for a computer, as the solid state drive can have advantages overhard drives in terms of performance, size, weight, ruggedness, operatingtemperature range, and power consumption. For example, SSDs can havesuperior performance when compared to magnetic disk drives due to theirlack of moving parts, which may avoid seek time, latency, and otherelectro-mechanical delays associated with magnetic disk drives.

Memory is also utilized as volatile and non-volatile data storage for awide range of electronic applications. Non-volatile memory may be usedin, for example, personal computers, portable memory sticks, digitalcameras, cellular telephones, portable music players such as MP3players, movie players, and other electronic devices. Memory cells canbe arranged into arrays, with the arrays being used in memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an apparatus in the form of acomputing system including at least one memory system, in accordancewith a number of embodiments of the present disclosure.

FIG. 2 illustrates a graph representing a number of threshold voltagedistributions and sensing voltages in accordance with one or moreembodiments of the present disclosure.

FIGS. 3A and 3B are tables illustrating mutual information for thresholdvoltage distributions in accordance with a number of embodiments of thepresent disclosure.

DETAILED DESCRIPTION

Apparatuses and methods for read threshold voltage selection areprovided. One example method can include setting a first soft readthreshold voltage and a second soft read threshold voltage based on adifference between a first number of memory cells that are read as beingprogrammed to a first state when read using a first threshold voltageand a second number of memory cells that are read as being programmed tothe first state when read using another threshold voltage.

Memory devices can include error correction circuitry (e.g.,implementing an error correction code (ECC)) to correct for errors insensing the data state of a memory cell. Hard data and soft data can beprovided to the error correction circuitry. Hard data is data thatcorresponds only to the data state of a memory cell. For example, a2-bit memory cell can be programmed to one of four data states, whereeach data state corresponds to one of hard data 00, 01, 10, or 11. Incontrast, soft data associated with a memory cell can indicate alocation of a state (e.g., threshold voltage (Vt)) stored on the memorycell within a distribution of states (e.g., Vt distribution)representing the target state to which the memory cell was programmed.Additionally, soft data associated with a memory cell can indicate aprobability of whether the state of the memory cell corresponds to thetarget state to which the memory cell was programmed. Some errorcorrection circuitry may operate more effectively with soft data.

In one or more embodiments of the present disclosure, soft data can beprovided from a memory device to error correction circuitry to correctdata read from a number of memory cells. The soft data can be obtainedby performing a number of sensing (e.g., read and/or program verify)operations. For example, a number of sensing operations can includeperforming two soft read operations and a hard read operation, whereinthe two soft read operations provide the soft data to the errorcorrection circuitry and the hard read provides the hard data to theerror correction circuitry. It can be desirable to provide soft datathat can be successfully used by the error correction circuitry tocorrect errors. Soft data that is generated during read operations usingsoft read voltage thresholds can result in greater amounts of mutualinformation and can be more likely to be used successfully by errorcorrection circuitry to correct errors. For example, soft data that isgenerated during read operations that use soft read voltage thresholdsthat result in greater amounts of mutual information is better forcorrecting errors via ECC than using soft read voltage thresholds thatresult in lesser amounts of mutual information of two adjacent thresholddistributions.

In a number of embodiments, soft read threshold voltages can be set,such that a particular percentage of memory cells read as being at aparticular data state during a hard read operation are located between ahard read threshold voltage and a soft read threshold voltage, toincrease the probability that the soft data generated during a soft readoperation can successfully be used by the error correction circuitry tocorrect errors. The particular percentage of memory cells read as beingat a particular data state during a hard read operation that are betweena hard read threshold voltage and a soft read threshold voltage can bebased on mutual information. For example, a first soft read thresholdvoltage can be set such that a read operation using the first soft readthreshold voltage generates an amount of mutual information that is atleast a particular percentage of an achievable peak mutual informationvalue.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how a number of embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure. As used herein, the designator “N” indicatesthat a number of the particular feature so designated can be includedwith a number of embodiments of the present disclosure.

As used herein, “a number of” something can refer to one or more of suchthings. For example, a number of memory devices can refer to one or moreof memory devices. Additionally, designators such as “N”, as usedherein, particularly with respect to reference numerals in the drawings,indicates that a number of the particular feature so designated can beincluded with a number of embodiments of the present disclosure.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. As will be appreciated,elements shown in the various embodiments herein can be added,exchanged, and/or eliminated so as to provide a number of additionalembodiments of the present disclosure. In addition, the proportion andthe relative scale of the elements provided in the figures are intendedto illustrate various embodiments of the present disclosure and are notto be used in a limiting sense.

FIG. 1 is a functional block diagram of a computing system 100 includingan apparatus in the form of at least one memory system 104, inaccordance with one or more embodiments of the present disclosure. Asused herein, an “apparatus” can refer to, but is not limited to, any ofa variety of structures or combinations of structures, such as a circuitor circuitry, a die or dice, a module or modules, a device or devices,or a system or systems, for example. In the embodiment illustrated inFIG. 1, the memory system 104 can include a controller 108 and one ormore memory devices 110-1, . . . , 110-N. In this example, thecontroller 108 is external to the one or more memory devices 110-1, . .. , 110-N. The memory devices 110-1, . . . , 110-N can provide a storagevolume for the memory system, e.g., with a file system formatted to thememory devices. The controller 108 can include control circuitry, e.g.,hardware, firmware, and/or software. In one or more embodiments, thecontroller 108 can be an application specific integrated circuit (ASIC)coupled to a printed circuit board including a physical interface andmemory devices 110-1, . . . , 110-N.

The controller 108 can include an error correction code (ECC) component112. The ECC component 112 can include hardware, firmware, and/orsoftware and can perform ECC operations on data, e.g., to correct errorsin the data. For example, errors in the state of a memory cell due tothreshold voltage shift can be corrected by ECC. ECC operations caninclude regular ECC operations used to correct errors based only on harddata and advanced ECC operations which can use soft data to correcterrors. Whether regular ECC and/or advanced ECC is used can depend onthe number of cells that are in error, for instance, e.g., a quantity oferroneous bits. Memory devices 110-1, . . . , 110-N can includecircuitry to determine a quantity of memory cells that change states,e.g., change from state 0 to state 1, between a first sensing operationperformed on a number of memory cells using a first sensing voltage anda second sensing operation performed on the number of memory cells usinga second sensing voltage. The quantity of memory cells that changestates between successive sensing operations can be used to set softread threshold voltages.

As illustrated in FIG. 1, a host 102 can be coupled to the memory system104. Host 102 can be a laptop computer, personal computers, digitalcamera, digital recording and playback device, mobile telephone, PDA,memory card reader, interface hub, among other host systems, and caninclude a memory access device, e.g., a processor. One of ordinary skillin the art will appreciate that “a processor” can intend one or moreprocessors, such as a parallel processing system, a number ofcoprocessors, etc.

In one or more embodiments, a physical host interface can be in the formof a standardized interface. For example, when the memory system 104 isused for data storage in a computing system 100, a physical hostinterface can be a serial advanced technology attachment (SATA),peripheral component interconnect express (PCIe), or a universal serialbus (USB), among other connectors and interfaces. In general, however, aphysical host interface can provide an interface for passing control,address, data, and other signals between the memory system 104 and ahost 102 having compatible receptors for the physical host interface.

The controller 108 can communicate with the memory devices 110-1, . . ., 110-N to read, write, and erase data, among other operations.Controller 108 can have circuitry that may be one or more integratedcircuits and/or discrete components. A controller could selectivelycouple an I/O connection (not shown in FIG. 1) of a memory device 110-1,. . . , 110-N to receive the appropriate signal at the appropriate I/Oconnection at the appropriate time. Similarly, the communicationprotocol between a host 102 and the memory system 104 may be differentthan what is required for access of a memory device 110-1, . . . ,110-N. Controller 108 can translate the commands received from a hostinto the appropriate commands to achieve the desired access to a memorydevice 110-1, . . . , 110-N.

A memory device 110-1, . . . , 110-N can include one or more arrays ofmemory cells, e.g., non-volatile memory cells. The arrays can be flasharrays with a NAND architecture, for example. Embodiments are notlimited to a particular type of memory device. For instance, the memorydevice can include RAM, ROM, DRAM, SDRAM, PCRAM, RRAM, and flash memory,among others.

The memory devices 110-1, . . . , 110-N can include a number of memorycells that can be grouped. As used herein, a group can include one ormore memory cells, such as a page, block, plane, die, an entire array,or other groups of memory cells. For example, some memory arrays caninclude a number of pages of memory cells that make up a block of memorycells. A number of blocks can be included in a plane of memory cells. Anumber of planes of memory cells can be included on a die. As anexample, a 128 GB memory device can include 4314 bytes of data per page,128 pages per block, 2048 blocks per plane, and 16 planes per device.

In a memory device, a physical page can refer to a unit of writingand/or reading, e.g., a number of cells that are written and/or readtogether or as a functional group of memory cells. An even page and anodd page can be written and/or read with separate writing and/or readingoperations. For embodiments including multilevel cells (MLC), a physicalpage can be logically divided into, for example, an upper page and alower page of data. For example, one memory cell can contribute one ormore bits to an upper page of data and one or more bits to a lower pageof data. Accordingly, an upper page and a lower page of data can bewritten and/or read as part of one writing and/or reading operation, asthe logical upper page and logical lower page are both part of the samephysical page.

The embodiment of FIG. 1 can include additional circuitry that is notillustrated so as not to obscure embodiments of the present disclosure.For example, the memory system 104 can include address circuitry tolatch address signals provided over I/O connections through I/Ocircuitry. Address signals can be received and decoded by a row decoderand a column decoder to access the memory devices 110-1, . . . , 110-N.It will be appreciated by those skilled in the art that the number ofaddress input connections can depend on the density and architecture ofthe memory devices 110-1, . . . , 110-N.

In general, the controller 108 is responsible for converting commandpackets received from the host 102, e.g., from a PCIe bus, into commandinstructions for host-memory translation circuitry and for convertingmemory responses into host system commands for transmission to therequesting host.

In one or more embodiments, data can be written to the memory devicesone page at a time. Each page in the memory device can have a number ofphysical sectors and each physical sector can be associated with alogical block address (LBA). As an example, a physical page can have 8physical sectors of data. However, embodiments are not limited to aparticular number of physical sectors per physical page.

FIG. 2 illustrates a graph representing a number of threshold voltagedistributions and sensing voltages in accordance with one or moreembodiments of the present disclosure. The example shown in FIG. 2represents one-bit, e.g., two-state, memory cells. However, embodimentsof the present disclosure are not limited to this example of one-bitmemory cells. Embodiments of the present disclosure can be used formemory cells that are programmed to any state and are part of any twoadjacent threshold voltage distributions.

As shown in FIG. 2, threshold voltage (Vt) distributions 220 and 222represent two states, e.g., L0 and L1, respectively, to which the memorycells can be programmed. State L0 can correspond to data 0 and state L1can correspond to data 1. However, embodiments are not limited to thesedata assignments.

Vt distributions 220 and 222 can represent a number of memory cells thatare programmed to the corresponding states, where the height of a Vtdistribution curve indicates a probability that a number of cellsprogrammed to a particular voltage within the Vt distribution, onaverage. The width of the Vt distribution curve indicates the range ofvoltages that represent a particular state, e.g., the width of the Vtdistribution curve 220 for L0 represents the range of voltages thatcorrespond to data 0.

A number of sensing voltages are illustrated in FIG. 2. Such sensingvoltages can include program verify voltages and/or read voltages, amongother sensing voltages. In the example illustrated in FIG. 2, voltagelevels 230 (R_(h)), 232-1 (R_(s1)), and 232-2 (R_(s2)) represent sensingvoltages, e.g., read voltages, that can be used to distinguish betweenstates L0 and L1 during a sensing operation. Voltage levels 230 (R_(h))can be a hard read threshold voltage and voltage levels 232-1 (R_(s1))and 232-2 (R_(s2)) can be soft read threshold voltages. In a sensingoperation performed on a selected memory cell in a NAND string, theunselected memory cells of the string can be biased with a pass voltageso as to be in a conducting state.

The sensed state of a memory cell can change, e.g., shift, over time dueto a number of mechanisms. For example, a charge storage structure,e.g., floating gate, of the memory cell may lose charge over time. Suchcharge loss can cause the Vt of the cell to change, e.g., decrease.Additionally, as the memory cell undergoes programming and/or sensingoperations over time, program disturb and/or read disturb mechanisms maycause the Vt of the cell to change, e.g., increase. Other mechanisms canalso cause the sensed state of a memory cell to change over time, aswill be appreciated by one of ordinary skill in the art.

In some instances, a Vt change can alter the sensed state of a memorycell. For example, if a memory cell were programmed to a target state ofL1, e.g., data 1, charge loss could cause the Vt of the memory cell todecrease to a level within Vt distribution 220 corresponding to stateL0, e.g., data 0. Accordingly, such a Vt change can result in erroneousdata being sensed during a sensing operation performed on the memorycell using the sensing voltages illustrated in FIG. 2, e.g., readvoltages R_(h), R_(s1), and R_(s2). For example, performing a sensingoperation using the sensing voltages illustrated in FIG. 2 may result ina determination that the memory cell represents a state other than thetarget state to which the cell was programmed. For instance, a sensingoperation performed on a memory cell that was programmed to a targetstate of L1 and has undergone charge loss may sense the state of thecell as L0, if hard read threshold voltage R_(h) is used in the sensingoperation. That is, using hard read threshold voltage R_(h) may resultin a cell programmed to store data 1 being erroneously sensed as storingdata 0.

As such, sensing voltages used during sensing operations performedbefore the Vt change occurs may no longer provide accurate and/orreliable sensing of memory cells that have undergone a Vt change, e.g.,charge loss. Therefore, it can be advantageous to determine soft dataassociated with the memory cells to account for Vt changes in Vtdistributions.

In a number of embodiments, it may be beneficial to use soft readthreshold voltages R_(s1), and R_(s2) that attempt to maximize themutual information. Mutual information can be calculated for memorycells comprising adjacent threshold voltage distributions, such as thethreshold voltage distributions (e.g., Vt distributions 220 and 222)shown in FIG. 2 and/or a normal distribution of threshold voltages. InFIG. 2, if the Vt of the cell is to the left of a read voltage, thewritten bit is decided as being at state “1,” and if the Vt of the cellis to the right of a read voltage, the written bit is decided as beingat state “0.” Therefore, if the Vt of a cell is to the left of R_(s1),the cell is determined to be a Strong 1 (S1) (e.g., 1 with highprobability). If the Vt of a cell falls between R_(s1) and R_(h), thecell is determined to be a Weak 1 (W1) (e.g., 1 with low probability).If the Vt of a cell falls between R_(h) and R_(s2), the cell isdetermined to be a Weak 0 (W0) (e.g., 0 with low probability). If the Vtof a cell is to the right of R_(s2), the cell is determined to be aStrong 0 (or 0 with high probability).

The status of the memory cells (e.g., S1, W1, W0, or S0) can be used todetermine mutual information for the memory cells in the two adjacentthreshold voltage distributions (e.g., Vt distributions 220 and 222).The probability that memory cells have a particular status is usedduring the determination of mutual information. In a number ofembodiments, mutual information can be determined based on a normaldistribution of threshold voltages or for a distribution of memory cellsrandomly written to a memory device. The following equation can be usedto denote an area under a normal distribution of threshold voltages:

${{Q(x)} = {\frac{1}{\left. \sqrt{}2 \right.\prod}{\int_{x}^{\infty}{e^{{- {t\;}^{2}}/2}{dt}}}}}\ $

The above equation can be used to determine the following probabilities,where Q for each of the read voltages is calculated and m denotes themean of the right side normal distribution of threshold voltages. Forexample, Prob (S1|0) is the probability that the determination is Strong1 given that 0 is written.

Pr(S1|1)=1−Q(R _(s1))

Pr(W1|1)=Q(R _(s1))−Q(R _(h))

Pr(W0|1)=Q(R _(h))−Q(R _(s2))

Pr(S0|1)=Q(R _(s2))

Pr(S1|0)=1−Q(R _(s1) −m)

Pr(W1|0)=Q(R _(s1) −m)−Q(R _(h) −m)

Pr(W0|0)=Q(R _(h) −m)−Q(R _(s2) −m)

Pr(S0|0)=Q(R _(s2) −m)

The probabilities from the above equations can be used in the equationsbelow to calculate mutual information (I) for the memory cells in thenormal distribution of threshold voltages. In the equations below, Xrepresents the state to which the memory cells were programmed and Yrepresents the state the memory cells at which the memory cells weredetermined to be at while performing the read operations using the readvoltages R_(h), R_(s1), and R_(s2).

H(Y|X=1)=−Pro(S1|1)log 2Prob(S1|1)−Prob(W1|1)log2Prob(W1|1)−Prob(W0|1)log 2Prob(W0|1)−Prob(S0|1)log 2Prob(S0|1)

H(Y|X=0)=−Pro(S1|0)log 2Prob(S1|0)−Prob(W1|0)log2Prob(W1|0)−Prob(W0|0)log 2Prob(W0|0)−Prob(S0|0)log 2Prob(S0|0)

H(Y|X)=P1*(Y|X=1)+P0*H(Y|X=0)

Pr(S1)=P0*Prob(S1|0)+P1*Prob(S1|1)

Pr(W1)=P0*Prob(W1|0)+P1*Prob(W1|1)

Pr(W0)=P0*Prob(W0|0)+P1*Prob(W0|1)

Pr(S0)=P0*Prob(S0|0)+P1*Prob(S0|1)

H(Y)=−Pro(S1)*log 2Prob(S1)−Prob(W1)*log 2Prob(W1)−Prob(W0)*log2Prob(W0)−Prob(S0)*log 2Prob(S0)

The mutual information (I) between X and Y can be computed as:

I(X;Y)=H(Y)−H(Y|X)

Mutual information can also be calculated for a group of memory cellsthat have been programmed to two adjacent states and read using readvoltages R_(h), R_(s1), and R_(s2). For example, the results of readoperations using R_(h), R_(s1), and R_(s2) can be:

Number Number Number Number Number of cells of cells of cells of cellsof cells written to read as read as read as read as a state Strong 1Weak 1 Weak 0 Strong 0 1 = 8961 14 8774 56 117 0 = 9186 1 1 1 9183

The probabilities can be calculated and used to determine mutualinformation, as shown below.

Prob(S1|1)=14/8961=0.0016

Prob(W1|1)=8774/8961=0.9791

Prob(W0|1)=56/8961=0.006

Prob(S0|1)=117/8961=0.0131

Prob(S1|0)=1/9186=1.0886e−4

Prob(W1|0)=1/9186=1.0886e−4

Prob(W0|0)=1/9186=1.0886e−4

Prob(S0|0)=9183/9186=0.9997

H(Y|X=1)=−Prob(S1|1)*log 2(Prob(S1|1))−Prob(W1|1)*log2(Prob(W1|1))−Prob(W0|1)*log 2(Prob(W0|1))−Prob(S0|1)*log2(Prob(S0|1))=−0.0016*log 2(0.0016)−0.9791*log 2(0.9791)−0.0062*log2(0.0062)−0.0131*log 2(0.0131)=0.1721

H(Y|X=0)=−Prob(S1|0)*log 2(Prob(S1|0))−Prob(W1|0)*log2(Prob(W1|0))−Prob(W0|0)*log 2(Prob(W0|0))−Prob(S0|0)*log2(Prob(S0|0))=−1.0886e−4*log 2(1.0886e−4)−1.0886e−4*log2(1.0886e−4)−1.0886e−4*log 2(1.0886e−4)−0.9997*log 2(0.9997)=0.0047

H(Y|X)=Prob(1)*H(Y|1)+Prob(0)*H(Y|0)=0.4938*0.1721+0.5062*0.0047=0.0874

Prob(S1)=Prob(1)*Prob(S1|1)+Prob(0)*Prob(S1|0)=0.4938*0.0016+0.5062*1.0886e−4=8.4504e−4

Prob(W1)=Prob(1)*Prob(W1|1)+Prob(0)*Prob(W1|0)=0.4938*0.9791+0.5062*1.0886e−4=0.4835

Prob(W0)=Prob(1)*Prob(W0|1)+Prob(0)*Prob(W0|0)=0.4938*0.0062+0.5062*1.0886e−4=0.0031

Prob(S0)=Prob(1)*Prob(S0|1)+Prob(0)*Prob(S0|0)=0.4938*0.0131+0.5062*0.9997=0.5125

H(Y)=−Prob(S1)*log 2(Prob(S1)−Prob(W1)*log 2(Prob(W1)−Prob(W0)*log2(Prob(W0)−Prob(S0)*log 2(Prob(S0)=−8.4504e−4*log2(8.4504e−4)−0.4835*log 2(0.4835)−0.0031*log 2(0.0031)−0.5125*log2(0.5125)=1.0356

I(X;Y)=H(Y)−H(Y|X)=1.0356−0.0874=0.9482

In the example above, the mutual information (I) for the group of memorycells programmed to two adjacent states and read using particular readvoltages is 0.9482. Mutual information for a group of memory cells canbe calculated for each set of read voltages that are used to during readoperations. For example, read operations can include a hard readthreshold voltage and two soft read threshold voltages. The aboveequations can be used to calculate mutual information on the results ofa particular combination of read voltages. In a number of embodiments, ahard read threshold voltage that corresponds to a voltage at theintersection of two adjacent threshold voltage distributions and twosoft read threshold voltages, one higher and one lower than the hardread threshold voltage, can be used during read operations.

In various embodiments, it can be beneficial to set the soft readthreshold voltages so that an amount of mutual information correspondingto the soft read threshold voltages is as great as possible. It can bedifficult and/or time consuming to determine the greatest amount ofmutual information that can be associated with two adjacent thresholdvoltage distributions. It is easier and less time consuming to determinea quantity of memory cells that are in a particular state (e.g., Strong1, Weak 1, Weak 0, Strong 0) for a particular set of read voltages.Therefore, the relationship between memory cells being at a particularstate and the corresponding mutual information for the memory cells canbe determined when the memory cells are read with a particular set ofread voltages.

In a number of embodiments, soft read threshold voltages can be selectedsuch that a particular percentage of memory cells read as being at aparticular data state during a hard read operation are between a hardread threshold voltage and a soft read threshold voltage. This increasesthe probability that the soft data generated during a soft readoperation can successfully be used by the error correction circuitry tocorrect errors. The particular percentage of memory cells read as beingat a particular data state during a hard read operation that are betweena hard read threshold voltage and a soft read threshold voltage can bebased on mutual information of two adjacent threshold voltagedistributions. For example, a first soft read threshold voltage can beset such that a read operation using the first soft read thresholdvoltage generates an amount of mutual information that is at least aparticular percentage of an achievable peak mutual information value fortwo adjacent threshold voltage distributions. In a number ofembodiments, the soft read threshold voltages can be set such thatmutual information is approximately 99% or 99.5% of an achievable peakmutual information value, for example. In one example, as will bedescribed below in association with FIGS. 3A and 3B, a number of memorycells that are a Weak 0 can be between 1% and 9% of a total number ofmemory cells read as Strong 0 and Weak 0 to ensure that mutualinformation is approximately 99% of an achievable peak mutualinformation value. In other embodiments example, a number of memorycells that are a Weak 0 can be between 1% and 5% of a total number ofmemory cells read as Strong 0 and Weak 0 to ensure that mutualinformation is approximately 99.5% of an achievable peak mutualinformation value. Therefore, soft read threshold voltages can be set sothat the number of memory cells that are a Weak 0 is between particularpercentages of a total number of memory cells read as Strong 0 and Weak0, such as 1% and 9% or 1% and 5% for example. The determination of thenumber of memory cells that are a Weak 0 can involve fewer calculationsand can be less time consuming than determining the mutual information.

In a number of embodiments, the soft read threshold voltages can bedetermined by calculating the hard read threshold voltage (e.g., thevoltages at the intersection of two adjacent threshold voltagedistributions). Read operations that use the hard read threshold voltageyield a minimum bit error rate. A read operation using the hard readthreshold voltage can be performed and a number of memory cells at aparticular state (n) (e.g., state 0) can be determined. The 1%, 5%, and9% values of n can be calculated. A number of soft read operations canbe performed using a number of soft read threshold voltages. The numberof memory cells at Weak 0 (m) can be determined for each of the numberof soft read operations. The number of memory cells at Weak 0 (p) is thedifference between the number of memory cells determined to be at state0 during the hard read operation and the number of memory cellsdetermined to be at state 0 during a soft read operation. A soft readthreshold voltage can be set at a particular voltage when the results ofa soft read operation including the particular voltage are:

x*n≦p≦y*n

wherein x and y are particular percentages. The particular percentage ofx and y can be any value. For example, x can be 1% and y can be 5% or9%. The particular percentage of x and y can be based on the mutualinformation for the memory cells read with a particular set of readvoltages.

FIGS. 3A and 3B are tables illustrating mutual information for thresholddistributions in accordance with a number of embodiments of the presentdisclosure. In FIGS. 3A and 3B mutual information is plotted against thepercentage of memory cells at Weak 0 at a number of soft read thresholdvoltages. In FIG. 3A, the maximum (e.g. peak) mutual information(MI_(max)) 340 for a group of memory cells from adjacent thresholdvoltage distributions is 0.9525. FIG. 3A also illustrates MI₉₉% 342-1and 342-2, where mutual information is at 99% of MI_(max). As shown inFIG. 3A at 342-1 and 342-2, if the mutual information for the group ofmemory cells is to be at least 99% of MI_(max), the soft read thresholdvoltages should be set so that the percentage of memory cell at Weak 0is between 1% and 9% of the total number of memory cells at state 0.

In the example of FIG. 3B, the maximum mutual information (MI_(max)) 340for a group of memory cells from adjacent threshold voltagedistributions is 0.9525. FIG. 3B also illustrates MI_(99.5)% 344-1 and344-2, where mutual information is at 99.5% of MI_(max). As shown inFIG. 3B at 342-1 and 342-2, if the mutual information for the group ofmemory cells is to be at least 99.5% of MI_(max), the soft readthreshold voltages should be set so that the percentage of memory cellat Weak 0 is between 1% and 6% of the total number of memory cells atstate 0.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of various embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of skill in the art uponreviewing the above description. The scope of the various embodiments ofthe present disclosure includes other applications in which the abovestructures and methods are used. Therefore, the scope of variousembodiments of the present disclosure should be determined withreference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

1. A method for read threshold voltage selection, comprising: setting a first soft read threshold voltage and a second soft read threshold voltage based on a difference between a first number of memory cells that are read as being programmed to a first state when read using a first threshold voltage and a second number of memory cells that are read as being programmed to the first state when read using another threshold voltage.
 2. The method of claim 1, wherein the method includes setting the first soft read threshold voltage at the another threshold voltage when the difference between the first number of memory cells and the second number of memory cells is between a first percentage of the first number of memory cells and a second percentage of the first number of memory cells.
 3. The method of claim 2, further including setting the first percentage at 1% and the second percentage at 9%.
 4. The method of claim 2, wherein further including setting the first percentage and the second percentage such that read operations using the first soft read threshold voltage yield a result of at least 99% of an achievable mutual information value for two adjacent threshold voltage distributions.
 5. The method of claim 2, further including the setting the first percentage at 1% and the second percentage at 5%.
 6. The method of claim 2, wherein further including setting the first percentage and the second percentage such that read operations using the first soft read threshold voltage yield a result of at least 99.5% of an achievable mutual information value for two adjacent threshold voltage distributions.
 7. The method of claim 1, comprising setting the second soft read threshold voltage at a second threshold voltage, wherein the difference between the first threshold voltage and the another threshold voltage is the same as a difference between the first threshold voltage and the second threshold voltage.
 8. A method for read threshold voltage selection, comprising: determining a first threshold voltage that corresponds to an intersection of a first threshold voltage distribution and a second threshold voltage distribution, wherein the first threshold voltage distribution corresponds to memory cells programmed to a first state and the second threshold voltage distribution corresponds to a memory cells programmed to a second state; performing a first read operation using the first threshold voltage; determining a first number of memory cells that are read as being programmed to the second state based on results of the first read operation; performing a second read operation using a second threshold voltage; determining a second number of memory cells that are read as being programmed to the second state based on results of the second read operation; and setting a first soft read threshold voltage at the second threshold voltage if the difference between the first number of memory cells and the second number of memory cells is between a first percentage of the first number of memory cells and a second percentage of the first number of memory cells.
 9. The method of claim 8, further comprising setting a second soft read threshold voltage at a third threshold voltage, wherein the difference between the first threshold voltage and the second threshold voltage is the same as a difference between the first threshold voltage and the third threshold voltage.
 10. The method of claim 8, further comprising setting a hard read threshold voltage at the first threshold voltage.
 11. The method of claim 8, further comprising performing a third read operation using a fourth threshold voltage if the difference between the first number of memory cells and the second number of memory cells is not between a first percentage of the first number of memory cells and a second percentage of the first number of memory cells.
 12. The method of claim 11, further comprising determining a third number of memory cells that are read as being programmed to the second state based on results of the third read operation.
 13. The method of claim 12, further comprising setting the first soft read threshold voltage at the fourth threshold voltage if the difference between the first number of memory cells and the third number of memory cells is between the first percentage of the first number of memory cells and the second percentage of the first number of memory cells.
 14. A method for read threshold voltage selection, comprising: setting a first soft read threshold voltage based on mutual information of two adjacent threshold voltage distributions, wherein the two adjacent threshold voltage distributions include a first threshold voltage distribution including memory cells programmed to a first state and a second threshold voltage distribution including memory cells programmed to a second state.
 15. The method of claim 14, wherein the method includes setting the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least a particular percentage of an achievable peak mutual information value for the two adjacent threshold voltage distributions.
 16. The method of claim 14, wherein the method includes setting the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least 99.5% of an achievable mutual information value for the two adjacent threshold voltage distributions.
 17. The method of claim 14, wherein the method includes setting the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least 99% of an achievable peak mutual information value for the two adjacent threshold voltage distributions.
 18. The method of claim 14, the method includes setting the first soft read threshold voltage as an offset from a hard read threshold voltage for the two adjacent threshold voltage distributions.
 19. The method of claim 14, the method includes setting a second soft read threshold voltage as an offset from a hard read threshold voltage for the two adjacent threshold voltage distributions, wherein the offset from the hard read threshold voltage is equal for the first soft read threshold voltage and the second soft read threshold voltage.
 20. An apparatus for read threshold voltage selection, comprising: an array of memory cells; and a controller coupled to the array of memory cells, the controller configured to: perform a first read operation using a hard read threshold voltage to determine a first number of memory cells that are read as being programmed to a first state; perform a second read operation using a first threshold voltage to determine a second number of memory cells that are read as being programmed to the first state; set a first soft threshold voltage as the first threshold voltage in response to the second number of memory cells being between a first percentage of the first number of memory cells and a second percentage of the first number of memory cells.
 21. The apparatus of claim 20, wherein the controller is configured to set a second soft threshold voltage at a second threshold voltage, wherein a difference between the first threshold voltage and the hard read threshold voltage is the same as a difference between the second threshold voltage and the hard read threshold voltage.
 22. The apparatus of claim 20, wherein the second number of memory cells being between the first percentage of the first number of memory cells and the second percentage of the first number of memory cells corresponds to an amount of mutual information from the second read operation being at least a particular percentage of an achievable peak mutual information value for two adjacent threshold voltage distributions.
 23. An apparatus for read threshold voltage selection, comprising: an array of memory cells; and a controller coupled to the array of memory cells, the controller configured to: set a first soft read threshold voltage based on mutual information of two adjacent threshold voltage distributions.
 24. The apparatus of claim 23, wherein the controller is configured to set the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least a particular percentage of an achievable peak mutual information value for the two adjacent threshold voltage distributions.
 25. The apparatus of claim 23, wherein the controller is configured to set a second soft read threshold voltage, wherein a difference between the first soft read threshold voltage and the hard read threshold voltage is the same as a difference between the second soft read threshold voltage and the hard read threshold voltage.
 26. The apparatus of claim 23, wherein the controller is configured to perform read operations using the first soft read threshold voltage and the second soft read threshold voltage and send results of the read operations to an error correction coding (ECC) component.
 27. The apparatus of claim 23, wherein the two adjacent threshold voltage distributions include a first threshold voltage distribution including memory cells programmed to a first state and a second threshold voltage distribution including memory cells programmed to a second state 